docs: def copy(self, old_name, new_name, index=None):
          """
          Copies an old sheet with the old_name to a new sheet with new_name.
  
          If an optional index argument is not provided then the created
          sheet is appended at the end. Returns the new sheet.
          """
          if index is None:
              index = len(self)
          self._copy(old_name, new_name, index)
          return self[new_name]